1. Field
The invention relates to reduction of water content in ligneous materials with high moisture content, particularly ligneous materials such as lignin solids separated from paper industry spent pulping liquor, such as spent waste alkaline digestion liquor known as “black liquor.”
2. State of the Art
U.S. Pat. Nos. 5,635,024 and 6,632,327, hereby incorporated by reference, describe a process for recovering lignin solids from spent, waste alkaline digestion liquors from the paper industry. These liquors are generally referred to as “black liquors.” The recovery of lignin solids is accomplished by mixing into spent digestion liquor a water soluble, surface active, polymeric, coagulant agent, and then acidifying the mixed liquor to a pH below 7 to separate the lignin and other organic compounds, and to coagulate them as solids which tend to float to the surface of the residual liquid. The coagulated solids, which generally form a lignin plastics solids mass are then separated from the residual liquid. These separated solids may contain up to about 85 percent liquid, mostly water. To make beneficial use of these solids, such as by combustion of the lignin solids as fuel for generating electric power and steam, it is generally desired to reduce the water content of these solids. To use these lignin solids as fuel for generating electric power and steam, the water content should be brought down to no more than about 30 percent water, and preferably to the range of less than about 5 percent water to gain the best possible thermal efficiencies.
The separated lignin solids can be dried to reduce the moisture content in commercial thermal treatment or drying equipment. However, this equipment uses substantial amounts of energy to dry the lignin solids, and therefore reduces the overall thermal fuel efficiency. To reduce energy consumed in drying, non-thermal means of reducing lignin solids moisture have been investigated such as mechanical separation of water from the lignin by pressure. It has been shown that lignin moisture content can be reduced to about 65 percent by compression in a pressure cell device, and subsequently dried by thermal methods using a commercial system to further reduce the moisture. However, this process still requires substantial drying energy. Even with the initial mechanical drying, however, the additional required drying by thermal methods involves high energy consumption to an extent almost equivalent to the energy values derived from combustion of the dried lignin. The significant energy needs in applying these past known techniques have discouraged their acceptance.
In another application, high moisture content sewage sludge collected from biological treatment, which sludge, in many respects, has properties similar to the lignin solids from black liquor, has been partially dewatered for combustion by filtration by the electrode watering technique. Again, however, the significant energy needs in reducing the water content of the sludge have discouraged use of sludge as an energy source. A simple and low energy cost method for processing lignin solids and similar materials such as sludge for water reduction is not known.